ND - Colour Vision Disorders III - Week 7

Question 1

What do photoreceptors truly do? What does this mean about how they interpret colour?

Answer 1

It can only count the number of photons it absorbs, it cannot tell wavelength
Single cones are colour blind
Multiple cones with different sensitivities are needed

Question 2

When colour matching, how many wavelengths do monochromats need?

Answer 2

Only one wavelength, and adjusting intensity

Question 3

What are the three kinds of monochromacy?

Answer 3

Typical (rod) monochromacy
Blue cone monochromacy
Atypical monochromacy

Question 4

Is typical monochromacy easy to diagnose? List 6 things to look out for.

Answer 4

Easy to diagnose
Lowered VA with no obvious explanation
Painless photophobia
Nystagmus
Reduced sensitivity to red light
Total colour blindness
Other affected family members

Question 5

What is VA like in typical monochromacy?

Answer 5

Equivalent to rod VA

Question 6

Consider a typical monochromat. Would their VA increase with increasing illiminances?

Answer 6

Up to a point, but will start decreasing again due to saturation

Question 7

List 5 characteristics of typical monochromcy photophobia.

Answer 7

Painless
Aversion to light
See better in dim light
Blink/squint/sunglasses
Photophobia is common but not invariable

Question 8

Is nystagmus universal in typical monochromacy? Explain.

Answer 8

No, it is common but not universal

Younger subjects seem to show it, but not older

Question 9

Are there any retinal changes with typical monochromacy?

Answer 9

None apparent

Question 10

What is the association between typical monochromacy and myopia?

Answer 10

Early reports of squinting and decreased viewing distance (to increase angular size of near work) erroneously lead to the notion of association wth myopia (there is no association)

Question 11

How do typical monochromats perform on pseudo-isochromatic plates?

Answer 11

Fail but irregular responses

Question 12

How do typical monochromats perform on Farnsworth D15?

Answer 12

On average, arrange in order of scotopic reflectance

Question 13

How do typical monochromats perform on Nagel anomaloscopes??

Answer 13

Full range but setting with increasing red because of scotopic spectral sensitivity

Question 14

What is the mode of inheritance for typical monochromacy?

Answer 14

Autosomal recessive

Question 15

List 5 findings for the mechanisms of typical monochromacy and list 2 implications of these findings.

Answer 15

Scotopic spectral sensitivity
Lowered VA
Preference for dim light
Nystagmus
-due to scotoma at rod free foveal area
-not a motor anomaly
These findings suggest:
-absence of cones
-cones not functioning

Question 16

What causes the nystagmus in monochromacy?

Answer 16

Scotoma at rod free foveal area

Question 17

What does anatomical evidence of monochromatic eyes suggest of the pure rod theory or rod only theory?

Answer 17

One consistency is the presence of morphologically intact cones, dismissing the pure rod theory

Question 18

Is a central scotoma found in all cases of monochromacy?

Answer 18

No

Question 19

Is there a rod-cone break in the following tests of monochromatic eyes?
Dark adaptation
Increment threshold
Critical fusion frequency
Directional sensitivity
What does this suggest about the cones functionally?

Answer 19

Yes to all, there was a break
This suggests that the cones are present, but contain rhodopsin or a pigment with a spectrum indistinguishable from rhodopsin

Question 20

How is blue cone monochromacy inherited?

Answer 20

X-linked recessive

Question 21

How does blue cone monochromacy compare to typical monochromacy?

Answer 21

Similar in all aspects including VA, nystagmus, and reduced red light sensitivity

Question 22

Do blue cone monochromats have some level of dichromacy?

Answer 22

Yes, but only rudimentary levels at mesopic levels

Question 23

Describe the characteristics of atypical (cone) monochromacy (3).

Answer 23

Normal VA
No photophobia
No nystagmus
Colour blind

Question 24

What could be a possible cause for atypical monochromacy?

Answer 24

A failure of the neural network rather than receptor dysfunction